The fabrication of high aspect ratio features in silicon is used extensively in the manufacture of micro-electro-mechanical-systems (MEMS) devices. Such features frequently have depths ranging from tens to hundreds of micrometers. To ensure manufacturability, the etching processes must operate at high etch rates to maintain reasonable throughputs. To ensure proper device performance, sidewall smoothness is often a critical requirement.
Conventional, single step, plasma etch processes cannot simultaneously meet these needs. As a result, a time division multiplex process has been developed where a deposition process is continuously alternated with an etch process. Each etch-deposition process pair constitutes a process cycle. The time division multiplexed (TDM) approach to etching silicon has been described by Laermer et. al. (U.S. Pat. No. 5,501,893, also known as the “Bosch” process). The TDM etch process is typically carried out in a reactor configured with a high-density plasma source, typically an Inductively Coupled Plasma (ICP) source, in conjunction with a radio frequency (RF) biased substrate electrode. The most common process gases used in the TDM etch process for silicon are sulfur hexafluoride and octofluorocyclobutane. Sulfur hexafluoride (SF6) is typically used as the etch gas and octofluorocyclobutane (C4F8) as the deposition gas. During the etch step, SF6 facilitates spontaneous and isotropic etching of silicon (Si); in the deposition step, C4F8 facilitates the deposition of the protective polymer layer onto the sidewalls as well as the bottom of the etched structures. The TDM alternating etch/deposition or “Bosch” process cyclically alternates between etch and deposition process steps enabling high aspect ratio structures to be defined into a masked silicon substrate. Upon energetic and directional ion bombardment, which is present in the etch steps, the polymer film coated in the bottom of etched structures from the previous deposition step will be removed to expose the silicon surface for further etching. The polymer film on the sidewall will remain because it is not subjected to direct ion bombardment, thereby, inhibiting lateral etching. While the TDM process consists of multiple etch-deposition cycles, either the etch or deposition (or both) portion of the cycle can be further divided into multi-step segments. Using the TDM approach allows high aspect ratio features to be defined into silicon substrates at high Si etch rates.
During the course of a conventional TDM etch process, it is known that process adjustments (known in the art as process morphing) are required to maintain vertical feature profiles, particularly for higher aspect ratios. Bhardwaj et. al. (U.S. Pat. No. 6,051,503) and Laermer et. al. (U.S. Pat. No. 6,284,148) teach increasing the deposition rate and/or decreasing the etch rate at the beginning of the process as solutions to this problem. Bhardwaj teaches that this deposition/etch rate adjustment can be accomplished by varying the process gas flows from cycle to cycle within the TDM process.
In a conventional TDM etch apparatus, gas introduction to the process chamber is controlled through a combination of mass flow controllers (MFCs) and isolation valves. During the etch segments, SF6 “on”, (SF6 is supplied to the process chamber) it is often beneficial (but not essential) to exclude the deposition gas (C4F8) from the process chamber. Similarly, during the deposition step, C4F8 “on”, (C4F8 is supplied to the process chamber) it is often beneficial (but not essential) to exclude the etch gas (SF6) from the process chamber.
It is known that turning an MFC on at the beginning of a process step results in a short pressure “burst” into the chamber until the MFC stabilizes to the setpoint value. For processes with longer step times, the effect of the pressure “burst” on the process is insignificant. However, as the process step times decrease, the pressure “burst” causes the process pressure to be out of compliance for a significant portion of the segment process time. For a TDM etch process, where the segment times are on the order of 5 seconds per segment, these repetitive pressure bursts adversely affect process reproducibility and stability. It is also known that holding the MFC at some low setpoint (near 1 sccm) in the “off” state, instead of a zero flow, improves stability. There is still a need, however, for a gas control system that facilitates stable system operation in processes that require fast repetitive gas composition changes.
One limitation of the TDM approach to etching is roughened feature side walls. This limitation is due to the periodic etch/deposition scheme used in a TDM etch process and is known in the art as side wall “scalloping”. For many MEMS device applications, it is desirable to minimize this side wall roughness or scalloping. The extent of scalloping in a TDM etch process is typically measured as a scallop length and depth. The scallop length is the peak-to-peak distance of the side wall roughness and is directly correlated to the etch depth achieved during a single etch cycle. The scallop depth is the peak to valley distance of side wall roughness and is correlated to the degree of anisotropy of an individual etch step. The extent of scallop formation can be minimized by shortening the duration of each etch/deposition cycle (shorter etch/deposition cycles repeated at a higher frequency), or by making each etch step more anisotropic in nature (e.g. allowing both the etchant and a small amount of the passivant to be present together in the etch step.)
In addition to smoother feature sidewalls it is also desirable to achieve higher overall TDM etch rates. The overall etch rate of a TDM etch process can be increased by either increasing the time spent in each etch cycle or increasing the efficiency within an etch cycle. Both of these approaches lead to larger scallop formation and ultimately rougher side walls. In a conventional TDM etch process, faster etch rates are only achievable at the expense of rougher side walls. Accordingly, there is a need for a high rate TDM etch process with smoother feature walls.
In a 1999 publication, Ayon et al. stated, without providing supporting information, “In general higher power to pressure ratios and shorter etching cycles tend to reduce this (scallop) effect.” It is well understood that at a given plasma power level, a lower pressure of SF6 results in a lower etch rate; and a lower pressure of C4F8 results in more polymer deposition which in turn leads to a lower etch rate. Though Ayon suggests shorter cycle times, the reported results only investigate the process space with deposition cycle times of 6 seconds or more and etch cycle times of 10 seconds or more without disclosing a method for controlling process gases on faster time scales.
In conventional TDM etch reactors, even when gas MFCs are held at a low setpoint during the “off” state, the response time of an individual MFC limits practical process segment times to greater than 2 seconds. Accordingly, there is a need for faster gas switching in TDM etching to achieve shorter process segment times.
A number of groups have reported deep silicon etch results using the TDM etch approach. The processes reported by these groups all used deposition cycle times of 4 seconds or longer while the reported etch times were 10 seconds or longer. Blauw et. al. report shorter cycle times with the TDM process through the use of fast mass flow controllers. Blauw reports a process using a 2 second deposition cycle in conjunction with a 5 second etch cycle. Blauw does not disclose a method for gas switching that allows the mass flow controllers to maintain a nearly constant flow of the etch and deposition gases during both the deposition and etch segments of the TDM process.
It is also known to minimize scalloping amplitude in a deep silicon etch by replacing the isotropic etching steps with anisotropic etching steps. For example, the addition of oxygen (O2) or nitrogen (N2) to the SF6 gas in the etch segments would slow down the etch rate at the sidewall (lateral etch rate) during the etch cycles. The reduced lateral etching is due to the formation of silicon oxide or nitride on the silicon sidewall. While this approach reduces scalloping, it is at the expense of the overall feature profile. Note, the formed oxide or nitride passivation layer is typically only a few monolayers thick which results in a process that is more difficult to control. U.S. Pat. No. 6,303,512 (Laermer et. al) addresses this limitation through the use of a SiF4/O2 based alternate process chemistry. One of the drawbacks of this technique is that the addition of oxygen-scavenger gases (such as CHF3, C4F8, CF4 etc.) to the plasma is needed to minimize oxide formation on the etch front—the bottom—to obtain the desired overall etch rate. The U.S. Pat. No. 6,303,512 patent does not disclose TDM cycle times faster than 5 seconds per segment or the use of a process gas bypass line to facilitate process segment cycle times shorter than 5 seconds.
Gas switching was disclosed in a time division multiplex etch patent (U.S. Pat. No. 5,501,893, Laermer et. al.). The U.S. Pat. No. 5,501,893 patent teaches gas switching at time scales near 1 minute per process segment, but does not teach the use of a gas bypass line to exhaust in conjunction with a shut-off valve as a means to quickly switch the process gases between the etch and deposition steps.
Suzuki et al. also disclose a gas switching method for TDM processes (U.S. Pat. No. 4,579,623). Suzuki teaches gas pulsing through the use of shut off valves in conjunction with a needle valve to maintain a constant gas flow. The shut off valves allow the gas to be either introduced to the process chamber or discharged to avoid pressure build up between the needle valve and the chamber shut off valve. As a consequence of using a needle valve in conjunction with gas switching, Suzuki's disclosure is limited to constant gas flow processes, which does not permit “morphing”. Suzuki does not teach gas switching that can be used for a TDM process where the gas flows are changed either within a cycle or cycle to cycle.
Suzuki also teaches discharging the gas between the needle valve and the shut off valve during the periods where the gas is not introduced into the vacuum chamber (the “off” cycle when gas is not introduced into the process). This discharge prevents pressure buildup between the needle valve and the shut off valve. Suzuki does not teach discharging the gas into the same chamber downstream of the reaction zone during the “off” cycle in order to present a more uniform gas load to the pumping system as the process gases cycle over time.
Furthermore, while Suzuki considers gas switching for plasma surface treatment processes that are cyclical and repeating, Suzuki does not teach gas switching for a TDM process that consists of alternating etching and polymerization steps.
Gas pulsing through the use of shut off valves in conjunction with mass flow controllers (MFCs) has been disclosed by Heinecke et. al. (U.S. Pat. No. 4,935,661). The Heinecke group teaches the use of a shut off valve after the process gas MFC in order to pulse process gases. Heinecke does not provide a bypass path for a process gas during the “off” state. Heinecke teaches leaving the MFC “on” while the shut off valve is in the “off” position and allowing the pressure to build in the line between the MFC and the valve. In this manner, when the valve is opened for the next process cycle using that gas, a pressure “burst” of that gas is released to the process chamber. Heinecke does not teach the use of a bypass path such that when the process gas is in the “off” state (not being introduced to the process chamber), the MFC remains “on” without a pressure buildup in the line. In addition, Heinecke does not teach the use of gas pulsing for cyclical etch/deposition processes.
Bhardwaj et. al. (U.S. Pat. No. 6,051,503) teaches gas switching for TDM etch processes where the gas flows are changed within a cycle or cycle to cycle. Bhardwaj does not teach the use of a gas bypass line to exhaust in conjunction with a shut-off valve as a means to quickly switch the process gases between the etch and deposition steps.
Van Suchtelen et. al (U.S. Pat. No. 4,916,089) teaches gas pulsing for epitaxial deposition. While Van Suchtelen teaches gas switching using a mass flow controller in conjunction with a gas bypass line, they do not teach the use of gas pulsing for etch processes, plasma-based processes, or cyclical etch/deposition processes.